A non-thermal plasma reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073, filed Dec. 16, 1999, entitled “Non-thermal Plasma Exhaust NOx Reactor,” which is hereby incorporated by reference herein in its entirety. Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field. The electrodes are coated onto alumina in a pattern that establishes a separati between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage.
U.S. Pat. No. 6,338,827 to Nelson et al., commonly assigned, entitled “Stacked Shape Plasma Reactor Design for Treating Auto Emissions,” which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element prepared from a planar arrangement of formed shapes of dielectric material. The shapes are used as building blocks for forming the region of the reactor wherein the plasma is generated. Individual cells are provided with a conductive print disposed on a formed shape to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential charge leakage. Multiple cells are stacked and connected together to form a multi-cell reactor element.
Commonly assigned U.S. patent application Ser. No. 09/517,681, filed Mar. 2, 2000 entitled “Plasma Reactor Design for Treating Auto Emissions—Durable and Low Cost,” which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element for conversion of exhaust gas constituents. The reactor comprises an element prepared from an extruded monolith of dense dielectric material having a plurality of channels separated by substantially planar dielectric barriers. Conductive material printed onto selected channels forms conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged. During operation, exhaust gas flows through channels and is treated by the high voltage alternating current plasma field. The planar shape of the dielectric barriers provides a uniform electrical response throughout the exhaust channels.
U.S. Pat. No. 6,354,903 to Nelson et al., commonly assigned, entitled “Method of Manufacture of a Plasma Reactor with Curved Shape for Treating Auto Emissions,” which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element wherein a swept shape substrate is formed and treated to create a non-thermal plasma reactor element. The substrate is formed via extrusion so that there is a series of nested, concentric dielectric barriers. Selected channels are coated with conductive material to form conductor channels capable of forming an electric field around exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged.
U.S. Provisional Application No. 60/249,231, of David E. Nelson, et al., filed Nov. 16, 2000, entitled “Edge-connected Non-thermal Plasma Exhaust After Treatment Device,” which is hereby incorporated by reference herein in its entirety, discloses an edge-connected non-thermal plasma reactor substrate including an edge-connected frame comprising a pair of dielectric edge connectors secured at opposite ends to first and second outer dielectric plates. The dielectric edge connectors comprise a backplane and a plurality of tines protruding along at least one major surface of the backplane, the plurality of tines being spaced apart from one another at regular intervals so as to form pockets between adjacent tines. A plurality of alternating polarity electrode plates are disposed within the edge-connected frame in an alternating polarity arrangement that defines the presence of at least one dielectric barrier next to a plasma cell with the pockets compliantly engaging opposite ends of the electrode plates.
While the above-described non-thermal plasma reactors meet some of the current needs and objectives in the art, there remain several issues that need to be more effectively addressed.
As illustrated in prior art FIGS. 1 and 2, elements for planar stacked plate non-thermal plasma reactor substrates are generally prepared by printing two individual dielectric (typically alumina) plates 10 with a metal conductor 12, drying each printed plate 14 and firing each printed plate 14 separately. Two separately fired printed plates 14 are stacked together having the metal conductors 12 sandwiched therebetween and stacked to form a two-plate substrate element 16 (exploded in FIG. 1 to show detail).
A multi-cell substrate 18, shown in FIG. 2, is prepared using a plurality of two-plate elements 16 having discrete spacers 20 placed so as to form multi-channel exhaust gas passages 22. Power and ground bus lines 24, 26 are established at the sides of the stack 18 to connect the alternating polarity two-plate elements 16 to power and ground connections (not shown). Exhaust gas passes through the exhaust gas passages 22 and is treated therein by the corona.
A problem arises with this construction method in that the two electrode printed plates 14 used to form the two-plate element 16 present a parting line gap (or conductor boundary line gap) 28 between the faying surfaces 30 of the two electrode plates 14 in the same plane as the metal conductor 12. The parting line gap 28 may be further increased by plate camber and thickness variation in the plates 10. Exhaust gas can penetrate the conductor boundary line gap 28 and contaminate the metal conductor 12. NOx conversion efficiency can be negatively affected even with a slight boundary gap due to gas by-pass between plates. Methods to prevent electrode contamination include, for example, coating the electrode surface with glass adhesive, bonding the two plates 14 together with glass adhesive and sealing the boundary line gap 28 with a sealant. However, such an approach increases processing costs and has unproven durability.
The dielectric plates 10 are very thin and fragile, having a typical plate 10 thickness in the range of about 0.25 millimeters to about 0.50 millimeters. Plate breakage can occur at high vehicle exhaust gas flow rates, especially under hard acceleration driving conditions. As high velocity gas passes through the narrow exhaust gas passages 22, Bernoulli-type lifting forces are generated toward the surfaces of the thin plates 10. This pressure causes the two-plate elements 16 to deflect and, in some cases, crack. The two-plate elements 16 may be strengthened by, for example, increasing the plate 10 thickness or reducing the parting line gap 28 span by using additional interior supports.
Further, the two-plate element 16 construction is prone to bus line arcing due to the parting line gap 28. Currently, several printing processes are employed to establish the bus lines 24, 26. A thick film metallic ink is used to fill the parting line gap 28 and make the connection between the two plates 14. As the reactor operates in a variable exhaust temperature range, thermal expansion and contraction of the ceramic plates 14 can cause the inked bus line 24, 26 to crack at the parting line, resulting in bus line arcing.
In an open space, opposite polarity plates must be separated from one another by a minimum of about 19 millimeters to prevent electrical arcing. Due to the parting line gap 28 existing between the two plates, current planar reactors separate the edge of the electrode from the edge of the dielectric plate 10 by about 19 millimeters to prevent arcing of one electrode to the opposite polarity bus line. This effectively reduces the potential active area of the electrode available to treat exhaust gas by the same amount.
Further, the fabrication of two-plate element 16 stacked planar reactors is complex and costly due to the substantial fixturing required to align pieces during assembly. For example, special fixturing is required to hold each discrete spacer in place relative to the plates.
What is needed in the art is an improved non-thermal plasma reactor substrate and process for preparing a non-thermal plasma reactor substrate.